A notable feature of the hippocampal network is the prevalent 5-12hz “theta” oscillation. This oscillation is easily observed in the local field potential of recording electrodes in the hippocampus, as in the figure to the right, as well is the phase histograms of interneurons and pyramidal cells throughout the hippocampus and entorhinal cortex. These histograms also illustrate that neurons in the hippocampus fire rhythmically.
The driving force for this oscillation is not fully understood, but appears to involve a combination of extrinsic oscillatory inputs from the medial septum in addition to intrinsic forces within the hippocampus. For an excellent review, see Buzsaki, Neuron 2002. You may also visit http://en.wikipedia.org/wiki/Theta_rhythm.